There are several reasons why monotheism was unusual in its time and place, but the main reason is that the major cultures such as Greece had been polytheistic.
In the historical establishment of any of the three monotheistic religions, there was always resistance, and the monotheism alone was perceived as something unusual. Beginning from Moses, through Jesus and his followers, to Muhammad, they were all accepted with reserve and were considered crazy by the majority. One of the reasons is that they were accustomed to the existing trade and lifestyle system, and the phenomenon of monotheism has shaken the trade system. Let us remember the Jews who made golden calf, merchants in Solomon's temple, or merchants from Mecca. The inert attitude of the people towards the new monotheism is influenced by the already existing polytheistic environment, the practical use of idols and gods, for harvest, rain, fertility, wars, etc. Another assumption that influences a heavy change in attitude in people is how they can believe in God who is invisible.
